# What Is MPPS in a HEPA Filter and Why Is It Tested?

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> Last updated: 2026-10-13 19:39:04
## What Is MPPS in a HEPA Filter?

MPPS stands for Most Penetrating Particle Size. It is the particle-size range at which a specific filter reaches its lowest efficiency and highest penetration under defined test conditions. MPPS is not a universal fixed value of 0.3 μm.

![Relationship between filtration efficiency and particle size](https://www.whalesens.com/storage/articles/mpps/mpps-curve.webp)

Particle capture is influenced by Brownian diffusion, interception, inertial impaction and electrostatic effects. Very small particles are strongly affected by diffusion, while larger particles are more readily captured by interception and inertial effects. The least efficient range may therefore occur between these regions.

## Why Measure MPPS?

Testing at MPPS helps identify the lowest point of a filter's efficiency curve. A single fixed test size may not represent the actual minimum efficiency of a particular filter. MPPS testing supports product verification, media comparison and efficiency classification.

The measured MPPS depends on media construction, face velocity, electrostatic condition, leakage and test method. The actual value should always be taken from the test result for the specific filter.

## MPPS and 0.3 μm

0.3 μm is a common reference size in engineering applications, but it is not automatically the MPPS of every filter. For reliable evaluation, review the complete efficiency-versus-particle-size curve or the relevant test report.

![Particle capture mechanisms](https://www.whalesens.com/storage/articles/mpps/body-02-capture-mechanisms-clean.webp)

## What Else Matters in Selection?

In addition to efficiency, consider rated airflow, face velocity, initial resistance, final resistance, sealing method and operating conditions. Both HEPA filters and liquid-tank HEPA filters require reliable installation sealing and leak testing.

For filter selection based on airflow, dimensions and cleanliness requirements, contact Whalesens.

Need help selecting a HEPA filter?

Contact the Whalesens engineering team for product selection and OEM/ODM support.

Email [whalesens@gmail.com](mailto:whalesens@gmail.com)

Contact Whalesens

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